The present invention relates generally to IC (integrated circuit) packages, and more particularly, to a method and system for determining the quality of bonding between a conductive ball and a conductive pad of an IC (integrated circuit) die within an IC package by etching the conductive ball from the conductive pad and analyzing a magnified image of the bottom of the conductive ball.
An IC (integrated circuit) die typically is housed within an IC (integrated circuit) package having leads that are coupled to conductive pads on the IC die for providing connection to nodes of the integrated circuit. Referring to FIG. 1, a cross sectional view of a lead frame strip 100 of an IC package includes an IC die 102 mounted on a die frame dap 104. The die frame dap 104 is coupled to the lead frame strip 100 via tie bars 108. The structures of the lead frame strip 100 are part of an IC package and are known to one of ordinary skill in the art of IC package manufacture.
Further referring to FIG. 1, a lead interconnect 112 of the lead frame strip 100 is coupled to a conductive pad on the IC die 102 for providing connection to a node of the integrated circuit of the IC die 102. A conductive ball 114 is bonded to a conductive pad 116 on the IC die 102. The conductive ball 114 is coupled to the lead interconnect 112 via a wire 118. FIG. 1 is a cross sectional view along a line Axe2x80x94A of a top view of the lead frame strip 100 of FIG. 2.
The performance of the integrated circuit of the IC die 102 is determined by the quality of bonding between the conductive ball 114 and the conductive pad 116. Referring to FIG. 3A, the conductive ball 114 is bonded to the conductive pad 116 using an ultrasonic and heating process to form an intermediary material 120 that bonds the conductive ball 114 to the conductive pad 116, as known to one of ordinary skill in the art of IC package manufacture. In the ultrasonic and heating process, the intermediary material 120 is formed from a first conductive material of the conductive ball 114 and a second conductive material of the conductive pad 116. For example, if the conductive ball 114 is comprised of gold and the conductive pad 116 is comprised of aluminum, then the intermediary material 120 is comprised of an intermetallic alloy (AuxAly) formed from the gold of the gold ball 114 and the aluminum of the aluminum pad 116.
FIG. 3A illustrates an example of poor bonding between the conductive ball 114 and the conductive pad 116. In FIG. 3A, a relatively low amount of material of the conductive ball 114 and the conductive pad 116 have been used to form a low amount of the intermediary material 120. With such low amount of intermediary material 120 bonding the conductive ball 114 to the conductive pad 116, the conductive ball 114 may have high resistance poor contact with the conductive pad 116. Such high resistance degrades the speed performance of the integrated circuit within the IC die 102.
FIG. 3B illustrates an example of good bonding between the conductive ball 114 and the conductive pad 116. In FIG. 3B, a relatively high amount of material of the conductive ball 114 and the conductive pad 116 have been used to form a high amount of the intermediary material 120. With such high amount of intermediary material 120 bonding the conductive ball 114 to the conductive pad 116, the conductive ball 114 may have low resistance good contact with the conductive pad 116. Such low resistance enhances the speed performance of the integrated circuit within the IC die 102.
Because the performance of the integrated circuit within the IC die 102 depends on the quality of bonding between the conductive ball 114 with the conductive pad 116, the quality of bonding is monitored during manufacture of IC packages. Referring to FIG. 4A, in the prior art, a cross section along line Bxe2x80x94B is made to result in the cross sectional view of FIG. 4B of the intermediary material 120 between the conductive ball 114 and the conductive pad 116. Such a cross sectional view is analyzed to determine the quality of bonding between the conductive ball 114 and the conductive pad 116. For example, a resin material is formed around the conductive ball 114 and the conductive pad 116, and the conductive ball 114 and the conductive pad 116 are polished from the side inward to the cross section Bxe2x80x94B to result in the cross sectional view of FIG. 4B.
For consistent analysis, the cross section Bxe2x80x94B should consistently be through the center of the conductive ball 114 as multiple IC packages are examined. However, polishing down to the exact center of the conductive ball 114 in the prior art is difficult to control. Referring to FIG. 5A, if the cross section Bxe2x80x94B is not at the center of the conductive ball 114, then the cross sectional view of FIG. 5B results with an inaccurate representation of the amount of intermediary material 120 between the conductive ball 114 and the conductive pad 116. Thus, the quality of the bonding between the conductive ball 114 and the conductive pad 116 cannot be consistently determined with the polishing method of the prior art.
In addition, a cross section of the intermediary material 120 may not be an accurate representation of the quality of bonding between the conductive ball 114 and the conductive pad 116. For example, if a void or a contaminant is present within the intermediary material 120, and if the cross section is not through such a void or contaminant, then the cross section alone may not represent the poor contact between the contact ball 114 and the contact pad 116 from such a void or contaminant.
Nevertheless, because bonding between the conductive ball 114 and the conductive pad 116 in an IC package determines the performance of the integrated circuit, accurate and consistent determination of the quality of bonding is desired.
Accordingly, in a general aspect of the present invention, the quality of bonding between a conductive ball and a conductive pad of a lead frame strip of an IC package is determined by etching the conductive ball from the conductive pad and analyzing the bottom of the conductive ball.
The conductive ball is comprised of a first conductive material and a conductive pad is comprised of a second conductive material. The conductive ball is bonded to the conductive pad by formation of an intermediary material formed from the first conductive material of the conductive ball and the second conductive material of the conductive pad.
In one embodiment of the present invention, the lead frame strip is immersed within an etching solution such that the intermediary material is etched between the conductive ball and the conductive pad until the conductive ball may be decoupled from the conductive pad. The conductive ball has a first color in areas where the intermediary material was not abutting the conductive ball and has a second color in areas where the intermediary material was abutting the conductive ball when the conductive ball was bonded to the conductive pad. The intermediary material was abutting an intermediary material area of the conductive ball when the conductive ball was bonded to the conductive pad. A magnified image of the intermediary material area on the conductive ball is captured by a microscope with the intermediary material area being substantially centered within the magnified image when the conductive ball is decoupled from the conductive pad.
The intermediary material area appears as the second color within the magnified image, and an area of the conductive ball wherein the intermediary material was not abutting the conductive ball appears as the first color within the magnified image. The magnified image of the intermediary material area on the conductive ball is analyzed by a data processor to determine quality of bonding between the conductive ball and the conductive pad.
The present invention may be used to particular advantage when the data processor calculates a percentage of the intermediary material area to a total area of the conductive ball within the magnified image to determine the quantity of bonding between the conductive ball and the conductive pad.
In another embodiment of the present invention, the etching solution is comprised of about 30% (weight/volume) sodium hydroxide (NaOH) at a temperature of about 80xc2x0 Celsius, and the lead frame strip is immersed in the etching solution for a time period of about 1 minute for optimized etching of the intermediary material between the conductive ball and the conductive pad.
In this manner, the quality of bonding between the conductive ball and the conductive pad may be consistently and accurately determined for a plurality of IC packages during manufacture of a high quantity of IC packages.
These and other features and advantages of the present invention will be better understood by considering the following detailed description of the invention which is presented with the attached drawings.